The pathways that go to the lateral geniculate nucleus will be studied by light and electron microscopical methods. Rats and cats will be used primarily to reveal the patterns of arborization formed within the nucleus by retinogeniculate, corticogeniculate, and tectogeniculate axons. The terminal arbors of these axons will be studied by filling them with horseradish peroxidase, and in addition they will be traced by autoradiographic and fiber degeneration methods. Changes occurring within these fiber systems will be studied by looking esppcially at the alterations produced in kittens by early monocular enucleations.